An accurate determination of the path of a device across a surface is important in a variety of applications. For example, if a faithful representation of an image of a scanned original is to be acquired, there must be accurate information as to the travel of the scanning device along the original. Typically, the captured image provided by a scanner is a pixel data array that is stored in memory in a digital format. A distortion-free image requires a faithful mapping of the original image to the pixel data array.
U.S. Pat. No. 5,149,980 to Ertel et al., which is assigned to the assignee of the present invention, describes use of a cross-correlation function to determine the relative movement between an original and an array of photoelements in a given direction. The patent notes that the one-dimensional approach can be extended to determine the vector of two-dimensional relative movement between the original and the array, so as to track translation, rotation and scaling in a two-dimensional plane.
The patent to Ertel et al. describes use of an optical sensor array to collect some form of "signature" of an original. The signature may be provided by illuminating and imaging the surface texture or other optical characteristics of the original. The light intensity will vary on a pixel-by-pixel basis with variations in surface texture. By cross-correlating images of the surface of the original, relative movement between the array and the original can be ascertained.
A critical element of the design of a system such as the one described by Ertel et al. is the circuitry which maintains the signal-to-noise ratio of each photoelement at a sufficiently high level to reliably determine the signature of the original. If the signal is the difference in reflectivity from pixel to pixel as a result of slight variations in paper texture of a white paper, the variations in reflectivity may be approximately six percent. If the sample rate objectives and the amount of possible subsequent signal averaging are considered, then noise terms in the signal must be less than the six percent paper reflectivity variation signal if useful information is to be obtained.
Thus, noise is one concern in the processing of signals from photoelements in a photoreceiver array. Another concern is manufacturing-induced variations in the performance of processing circuitry. Operations that rely upon computations of the differences in reflectivity from one pixel to another pixel are susceptible to error if electrically parallel transfer circuits vary in performance. In the ideal, there are no pixel-to-pixel signal differences due to variations in signal transfer circuit performance, so that differences between pixel signals are solely attributable to differences in light reception at the photoelements. However, circuit devices vary in performance, even when the devices are formed by the same fabrication process.
Yet another concern involves reliably obtaining useful information regarding an imaged surface when there are image-affecting considerations that are consistent among pixels in one neighborhood of pixels, but different among pixel neighborhoods. For example, illumination optics may create consistent neighborhood-to-neighborhood variations in the illumination of the surface to be imaged. Non-uniform illumination will cause artifacts. Another example of a consistent localized pattern is one in which a portion of the photoelement array is directed at a surface area having a dark background, while the remainder of the photoelements are directed at an unshaded area of the surface.
What is needed is transfer circuitry for connection to a photoreceiver arrangement, with the circuitry having a reduced susceptibility to errors caused by manufacturing-induced variations of devices and by multiple-pixel patterns of photoelement signal generation and/or processing.